OTA Feedback Mechanism for Fixed Feedback Voltage Regulators

ABSTRACT

An operational transconductance amplifier used in conjunction with a multiple chip voltage feedback technique allows multiple strings of LEDs and current sinks to be efficiently powered by a simple feedback oriented voltage regulator within an appliance. A connected series of differential amplifiers or multiplexors are used to monitor the voltages between the connected LEDs and the current sinks, in order to progressively determine the lowest voltage. The operational transconductance amplifier compares this voltage to a reference voltage and injects or removes current from the feedback node of a voltage regulator, thereby altering the voltage present at the feedback node. This causes the voltage regulator to adjust its output, ensuring that the current sinks of the LED strings have adequate voltage with which to function, even as the LEDs have different forward voltages and the strings are asynchronously enabled and disabled.

BACKGROUND OF THE INVENTION

Modern televisions employ many types of voltage regulators in order togenerate various power supplies within the television itself. Theseoff-the-shelf power supplies have characteristics that are known anddesired vis-à-vis the ways that they perform and interact with othercomponents within the television. Television manufacturers arecomfortable with the regulators that they have employed in the past, andcan be reluctant to change out this critical part.

Often, these voltage regulators rely on a resistor divider feedbacksignal in order to regulate their output voltages, as in FIGS. 1, 2, and3. (These figures show the power consumer as the simple resistorR_(Load), but the power consumption may be much more complex.) Incircuits such as these, as power consumption increases the converter'soutput voltage naturally falls, the feedback voltage falling along withit. As the output voltage and the feedback voltage fall, the discrepancyin the constant comparison between the feedback voltage and a knownvoltage within the television or the voltage regulator itself revealsthe increased power demand, which in turn causes the power supply toincrease its power output. The voltage rises, the feedback voltagerises, and the system heads in the direction towards equilibrium. Theprocess works conversely as power consumption decreases.

Consider the simple case illustrated in FIG. 4. The simplified displayillustrated in FIG. 4 consists of four strings of ten LEDs per string,each string capable of being switched on and off independently of theothers. Each string also contains a current sink, ensuring that eachilluminated string receives the same amount of current as each otherstring. This ensures that each illuminated LED produces the samelight—in both intensity and color—as all of the others.

Each illuminated LED requires a forward voltage of approximately 3.5volts, and the current sink requires 1.2 volts in order to operate.Allowing for the vagaries inherent in the LED manufacturing process,each serially connected string requires about 36.2 volts for the 10 LEDsand the current sink. Because the voltage output is (in this case)fixed, in order to ensure sufficient voltage for the operation of thecurrent sinks given the variableness of the LEDs, it would be preferableto allocate about 40 volts.

The typical current that would be desired across the feedback circuitrywould be 100 microamps, implying a total resistance (R1+R2) of 400Kohms. If the feedback voltage that the 40 volt regulator requires is 2.4volts, we'd use resistors of 376,000 ohms and 24,000 ohms to divide thedesired 40.0 volt output into the required 2.4 volts. As the stringsswitch on and off, the power required from the regular goes up and downas the regulator keeps the LEDs lit.

There are some real life problems with the way that this circuitaccomplishes the task of keeping the lights on. For example, the desiredoutput voltage may not be 40.0 volts. Consider the “average” string of10 LEDs with the “average” total forward voltage of 35.0 volts. Combinedwith the 1.2 required voltage drop across the current sink, the totalrequired voltage is only 36.2 volts. With a 40.0 volt supply, all of theextra 3.8 volts worth of power is wasteful (and problematic) heat,dissipated in this example across the current sink. By considering a“worst case LED scenario” rather than an “actual requirement” scenario,excess power is generated and dissipated.

In addition, measuring the voltage at the “top” of the strings is notoptimal—a better method would be to measure the feedback voltage abovethe current sinks as is pictured in FIG. 5, not above the LEDs as isimplicit in FIG. 4. Measuring the voltage across the current sinks,where the excess voltage “accumulates,” is a better way to determine therequired voltage output from the regulator—the circuit should optimallyensure that there's enough voltage (1.2 volts in this example) acrosseach current sink, not that the LED string sees a fixed voltage. Onecomplication with this methodology is that the circuit needs to knowwhich current sinks are on and which are off at any particular time, asit should only ensure adequate voltage across the “on” current sinks.

And finally, there may be quite a few strings of LEDs, making itdifficult to use one integrated circuit to perform the “minimum voltage”comparison. Though FIGS. 4 and 5 show ten strings of LEDs, a typicallarge television might have one hundred or more strings. It would bepreferable to have a solution that scales across a large number ofstrings, a solution where that comprises a number of control andcomparison chips that are linked together rather than oneextraordinarily large comparison chip.

What is needed is a method for adapting these legacy voltage regulatorsfor use in systems with variable voltage requirements that must bemeasured in a number of different places within the circuitry.

SUMMARY OF THE INVENTION

The invention provides a method for manipulating the feedback voltageinput into a legacy voltage regulator in order to direct the converterto alter its output voltage. It is scalable to allow its application indifferent devices that might contain widely varying numbers of LEDstrings.

The invention comprises two important parts that work with the legacyvoltage regulator. First, the invention uses a series of seriallyconnected integrated circuits (herein referred to as controllers) totabulate the current “lowest voltage.” The first controller measures aset number of voltages, determines the lowest voltage from thesemeasurements, and then passes that lowest voltage to the next controllerin the series. Each successive controller compares each of itsmeasurements to each other and the one lowest measurement from theprevious controller and passes the “new” lowest voltage to the nextcontroller in the series. The output of the final controller is then thelowest voltage in from the set of all of the voltages that are beingcompared. Assuming that the lowest voltage is above the requiredvoltage, the voltage regulator is producing sufficient power to operatethe LEDs.

One extension to the invention allows the serially connected controllersto consider statuses as well as voltages. This distinction is usefulwhen a particular LED string might be off, and thus have an irrelevantvoltage. In this case, the extended invention would allow the irrelevantvoltage to be effectively ignored. For example, if the statuses of thestrings were either ACTIVE or INACTIVE, the various controllers wouldconsider only the voltages on the ACTIVE strings as the lowest voltageis tabulated from one controller to the next. By passing an ACTIVEstatus to the succeeding controller, each controller can indicate thatat least one of its monitored strings—or at least one monitored stringfrom a preceding controller—was ACTIVE and had a correspondingly usefulvoltage.

The invention also comprises an operational transconductance amplifier,or OTA. The OTA compares the lowest voltage from the series of seriallyconnected integrated circuits to the known voltage required for thecurrent sinks to operate. Then, it produces an output current that isproportional to the difference and injects that current into thefeedback mechanism of the voltage regulator. When the lowest voltage isabove the required voltage, the OTA produces current that raises thefeedback voltage, causing the voltage regulator to lower its poweroutput. The OTA can be tuned for specific applications, and generally,the higher the difference, the more current the OTA produces.

Conversely, when the voltage regulator is producing insufficient power,and the voltage across the current sinks drops to levels below therequired voltage, the OTA can remove current from the feedbackmechanism, causing the voltage regulator to raise its power output. Thehigher the deficit, the more current removed and the lower the feedbackvoltage becomes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a voltage regulator feedback resistor network with nofrequency compensation.

FIG. 2 shows a voltage regulator feedback resistor network with feedforward frequency compensation.

FIG. 3 shows a voltage regulator feedback resistor network with type IIIfrequency compensation.

FIG. 4 shows a voltage regulator powering 4 strings of LEDs.

FIG. 5 shows a voltage regulator powering 4 strings of LEDs wherein thepertinent feedback points are indicated.

FIG. 6 shows a first embodiment of the invention with no frequencycompensation in the feedback resistor network.

FIG. 7 shows the construction of a typical operational transconductanceamplifier.

FIG. 8 shows a graph of current versus voltage differential for atypical operational transconductance amplifier.

FIG. 9 shows a second embodiment of the invention with feed forwardfrequency compensation in the feedback resistor network.

FIG. 10 shows a third embodiment of the invention with type IIIfrequency compensation in the feedback resistor network.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 6 shows a schematic of an embodiment of the invention that containsone voltage regulator, one operational transconductance amplifier, threeintegrated circuits, each integrated circuit controlling eight stringsof ten LEDs per string, and three resistors used to control both thefeedback mechanism of the voltage regulator and the interaction betweenthe voltage regulator and the operational transconductance amplifier. Inthis embodiment, much of the functionality of measuring the voltages,comparing the voltages is contained within the three integratedcircuits. These three integrated circuits also contain the current sinksthat control the current through the LEDs—one current sink per string.This embodiment would be contained within a television that uses thestrings of LEDs as a backlight device.

The integrated circuits also control, via commands from the enclosingtelevision, each of the 24 strings of LEDs, turning them on or off, ordimming them via a PWM mechanism internal to the individual integratedcircuits, as requested by the television. The television can use any ofa number of different methods to communicate with the integrated circuitas it pertains to the control of the current sinks. In this embodiment,a simple one-wire, serial interface is used.

The LED strings are composed of LEDs that have forward voltages ofapproximately 3.5 volts per LED, so each string of ten LEDs will have aforward voltage of approximately 35 volts. The current sinks that arepart of the three integrated circuits, one current sink per LED string,typically require approximately 1.2 volts to function, so the totalapproximate required voltage for each channel, composed of one, ten LEDstring and one current sink, is approximately 36.2 volts. LEDmanufacturing process variations and the placement of particular LEDs onparticular strings cause variations in the required voltages fromchannel to channel, so the range in the case of this television could beas wide as four volts, from 34 to 38 volts, but it is the voltage acrossthe current sinks (the 1.2 required volts) that is important here. Thethree integrated circuits communicate serially to determine the lowestvoltage across an individual current sink on any of the active LEDstrings, with the last integrated circuit in the series returning thelowest voltage to the operational transconductance amplifier.

The regulator is a “typical” 24 volt to 40 volt DC-to-DC converter,though the embodiment here will require only 36.2 volts of output.External to the drawing is an AC-to-DC converter whose output is 24volts DC.

The typical current across the feedback circuitry of the DC-to-DCconverter would be 100 microamps, implying a total resistance (R1+R2) of362K ohms. The desired feedback voltage—when the DC-to-DC converter isproducing a minimum of 1.2 volts across any of the active currentsinks—would be 2.4 volts, resulting in resistor values of 338K ohms forR1, and 24K ohms for R2.

The operational transconductance amplifier compares the lowest voltageacross any of the active current sinks (the voltage returned by thethree integrated circuits) to a reference voltage that corresponds tothe minimum voltage required to power a current sink (VREG—in this case,1.2 volts) and outputs a current that is proportional to the differenceas is shown in FIG. 8. The OTA would have a maximum current output ofImax+ and a minimum output of Imax−. The OTA is built via FIG. 7.

In this embodiment, the “current bounds” of the OTA (FIG. 8, Imax+ andImax−) are higher than the network of R1 and R2, and the DC-to-DCconverter can manage. In some situations, using a pre-existing, alreadydesigned OTA within this circuit could feed too much positive ornegative current into the surrounding circuit, overpowering the feedbackvoltage beyond the ability of the DC-to-DC converter to be able tomanage. That is, the feedback mechanism of the DC-to-DC converter in thecircuit could be overwhelmed by the OTA's Imax+ and Imax− current. R3 isinstalled in the circuit in order to eliminate this possibility, theOTA's ability to drive the feedback past the ability of the DC-to-DCconverter to manage. The ratio of R3 to R1 and R2 will define the boundsof the OTA's ability to raise and lower VFB, but a typical value for R3in his situation would be 140K ohms.

This embodiment also contains a number of related components thatprovide context within which the invention operates. The televisioncontains an internal power supply, typically an AC-to-DC supply thatprovides a specific voltage output. In the case of this embodiment, thepower supply provides 24 volts, though other AC-to-DC power supplies areoften found in televisions, providing DC voltage outputs that are bothhigher and lower than the voltage required by the LED strings.

The DC-to-DC converter requires 3 major inputs: the 24 volt input fromthe television's power supply, the reference voltage from the televisionthat indicates the voltage required for the current sinks to operate asexplained previously, and a feedback voltage that indicates the minimumvoltage being supplied to the current sinks.

Other embodiments are possible. FIG. 9 shows another embodiment whereinthe feedback mechanism of the voltage regulator compromises feed forwardfrequency compensation in the feedback resistor network. FIG. 10 showsanother embodiment wherein the feedback mechanism of the voltageregulator additionally comprises type III frequency compensation in thefeedback resistor network.

Presuming that the television has an AC source as its ultimate powersource, the decision to use a single AC-to-DC (110V AC to 40V DC)voltage regulator as opposed to a combination of one AC-to-DC regulator(to convert 110V AC to 24V DC) and then one DC-to-DC regulator (toconvert 24V DC to 40V DC) would be the television manufacturer's tomake. The invention functions similarly in either case, with the OTAfeedback mechanism connected to the regulator that directly powers theLEDs. (In the case of a two regulator system, there is generally anadditional feedback mechanism on the “outer” AC-to-DC regulator. Thatfeedback mechanism could be integral to the AC-to-DC converter.)

In addition, it would be possible to bypass the combination of anAC-to-DC converter and a DC-to-DC converter altogether, insteadutilizing a power supply that took a wall voltage (for example, 120 VAC)and converted it directly into the 40 volts required by the television,assuming that the power supply utilized the voltage feedback mechanismdescribed here.

1. A circuit that comprises: two or more serially connected controllerswhere: the first controller is configured to set a summary voltage to beequal to one of a set of voltages monitored by that controller; and eachnon-first controller is configured to set the summary voltage to beequal to either the summary voltage generated by the precedingcontroller in the series or one of a set of voltages monitored by thatcontroller and; an operational transconductance amplifier configured togenerate an output current in proportion to the difference between thesummary voltage established by the final controller and a second voltageinput; and a DC-to-DC voltage regulator configured to regulate an outputvoltage in response to a feedback voltage; and a means for utilizing thecurrent produced by the operational transconductance amplifier to alterthe feedback voltage of the voltage regulator.
 2. A circuit as in claim1 wherein the second measurement input of the operationaltransconductance amplifier is configured to be a fixed referencemeasurement.
 3. A circuit as in claim 1 that also comprises a feedbacknetwork comprising two resistors, wherein said feedback network isconnected to the output voltage of the voltage regulator, the feedbackvoltage connection of the voltage regulator, and the output currentconnection of the operational transconductance amplifier.
 4. A circuitas in claim 3 wherein the feedback network further comprises a thirdresistor and a capacitor.
 5. A circuit as in claim 4 wherein thefeedback network further comprises a fourth resistor, a secondcapacitor, and a third capacitor.
 6. A circuit as in claim 1 wherein:the first controller is further configured to set a summary status to beequal to one of a set of statuses monitored by that controller; and eachnon-first controller is further configured to set the summary status tobe equal to either the summary status generated by the precedingcontroller in the series or one of a set of statuses monitored by thatcontroller.
 7. A circuit as in claim 6 wherein the second voltage inputof the operational transconductance amplifier is configured to be afixed reference measurement.
 8. A circuit as in claim 6 that alsocomprises a feedback network comprising two resistors, wherein saidfeedback network is connected to the output voltage of the voltageregulator, the feedback voltage connection of the voltage regulator, andthe output current connection of the operational transconductanceamplifier.
 9. A circuit as in claim 8 wherein the feedback networkfurther comprises a third resistor and a capacitor.
 10. A circuit as inclaim 9 wherein the feedback network further comprises a fourthresistor, a second capacitor, and a third capacitor.
 11. A circuit thatcomprises: two or more serially connected controllers where: the firstcontroller is configured to set a summary voltage to be equal to one ofa set of voltages monitored by that controller; and each non-firstcontroller is configured to set the summary voltage to be equal toeither the summary voltage generated by the preceding controller in theseries or one of a set of voltages monitored by that controller and; anoperational transconductance amplifier configured to generate an outputcurrent in proportion to the difference between the summary voltageestablished by the final controller and a second voltage input; and anAC-to-DC voltage regulator configured to regulate an output voltage inresponse to a feedback voltage; and a means for utilizing the currentproduced by the operational transconductance amplifier to alter thefeedback voltage of the voltage regulator.
 12. A circuit as in claim 11wherein the second measurement input of the operational transconductanceamplifier is configured to be a fixed reference measurement.
 13. Acircuit as in claim 11 that also comprises a feedback network comprisingtwo resistors, wherein said feedback network is connected to the outputvoltage of the voltage regulator, the feedback voltage connection of thevoltage regulator, and the output current connection of the operationaltransconductance amplifier.
 14. A circuit as in claim 13 wherein thefeedback network further comprises a third resistor and a capacitor. 15.A circuit as in claim 14 wherein the feedback network further comprisesa fourth resistor, a second capacitor, and a third capacitor.
 16. Acircuit as in claim 11 wherein: the first controller is furtherconfigured to set a summary status to be equal to one of a set ofstatuses monitored by that controller; and each non-first controller isfurther configured to set the summary status to be equal to either thesummary status generated by the preceding controller in the series orone of a set of statuses monitored by that controller.
 17. A circuit asin claim 16 wherein the second voltage input of the operationaltransconductance amplifier is configured to be a fixed referencemeasurement.
 18. A circuit as in claim 16 that also comprises a feedbacknetwork comprising two resistors, wherein said feedback network isconnected to the output voltage of the voltage regulator, the feedbackvoltage connection of the voltage regulator, and the output currentconnection of the operational transconductance amplifier.
 19. A circuitas in claim 18 wherein the feedback network further comprises a thirdresistor and a capacitor.
 20. A circuit as in claim 19 wherein thefeedback network further comprises a fourth resistor, a secondcapacitor, and a third capacitor.
 21. A method of regulating the outputvoltage of a DC-to-DC voltage regulator in response to voltagesmonitored by two or more controllers, the method comprising: monitoring,by each controller, a set of respective voltages; selecting, by thefirst controller, one summary voltage from a set of voltages monitoredby the first controller; selecting, by each following controller, onesummary voltage to be equal to either the summary voltage generated bythe preceding controller in the series or one voltage from the set ofvoltages monitored by that controller; generating, via an operationaltransconductance amplifier, a current that is proportional to thedifference between the summary voltage generated by the final controllerand a second input voltage; altering, via the output current of theoperational transconductance amplifier, the feedback voltage of avoltage regulator; and regulating the output voltage of the voltageregulator in response to the feedback voltage.
 22. A method as in claim21 that further comprises: selecting, by the first controller, onesummary status from a set of statuses monitored by the first controller;selecting, by each following controller, one summary status to be equalto either the summary status generated by the preceding controller inthe series or one status from the set of statuses monitored by thatcontroller.
 23. A method of regulating the output voltage of an AC-to-DCvoltage regulator in response to voltages monitored by two or morecontrollers, the method comprising: monitoring, by each controller, aset of respective voltages; selecting, by the first controller, onesummary voltage from a set of voltages monitored by the firstcontroller; selecting, by each following controller, one summary voltageto be equal to either the summary voltage generated by the precedingcontroller in the series or one voltage from the set of voltagesmonitored by that controller; generating, via an operationaltransconductance amplifier, a current that is proportional to thedifference between the summary voltage generated by the final controllerand a second input voltage; altering, via the output current of theoperational transconductance amplifier, the feedback voltage of avoltage regulator; and regulating the output voltage of the voltageregulator in response to the feedback voltage.
 24. A method as in claim23 that further comprises: selecting, by the first controller, onesummary status from a set of statuses monitored by the first controller;selecting, by each following controller, one summary status to be equalto either the summary status generated by the preceding controller inthe series or one status from the set of statuses monitored by thatcontroller.